SIPOS, a material comprising polycrystalline silicon and oxygen, is commonly used to passivate the surfaces of a semiconductor device. The physical properties of this material are elaborated upon in U.S. Pat. No. 4,014,037, SEMICONDUCTOR DEVICE, T. Matsushita et al., Mar. 22, 1977; in "Crystallographic study of semi-insulating polycrystalline silicon (SIPOS) doped with oxygen atoms", M. Hamasaki et al., J. Appl. Phys. 49(7), July 1978, pp. 3987-3992; and in "Electronic Properties of Semi-Insulating Polycrystalline-Silicon (SIPOS) Doped with Oxygen Atoms", M. Hamasaki et al., Solid State Communications, Vol. 21, No. 6, 1977, pp. 591-593.
SIPOS is typically deposited in a chemical vapor deposition (CVD) system and it is frequently deposited at less than atmospheric (i.e., at "low") pressure. Examples of such low-pressure chemical vapor deposition (LPCVD) systems can be found in U.S. Pat. No. 3,900,597, SYSTEM AND PROCESS FOR DEPOSITION OF POLYCRYSTALLINE SILICON WITH SILANE IN VACUUM, J. L. Chruma et al., Aug. 19, 1975; in "An Analysis of LPCVD System Parameters for Polysilicon, Silicon Nitride and Silicon Dioxide Deposition", W. A. Brown et al., Solid State Technology, July 1979, pp. 51-58; and in "Low Pressure CVD Production Processes for Poly, Nitride, and Oxide", R. S. Rosler, Solid State Technology, April 1977, pp. 63-70.
The method of the present invention can be performed in conventional low pressure deposition systems. These systems typically comprise a tube-shaped deposition chamber which is axially surrounded by heater elements, a reactant gas inlet portion at one end of the deposition tube, and an exhaust gas outlet portion at the opposite end of the deposition tube. Wafers on which the deposition is to be formed are serially arranged between the inlet portion and the outlet portion.
During the deposition process, a reactant gas (or mixture of reactant gases) is admitted to the deposition tube through the inlet portion, flows over the wafers within the tube, and exhausts through the outlet portion. To compensate for depletion of certain components of the reactant gas(es) along the length of the deposition tube, the power to the heater elements which surround the tube may be adjusted so as to provide a particular temperature profile.
For example, when depositing polycrystalline silicon from a silane reactant gas, the silane depletes as it flows from the inlet portion to the outlet portion. If the temperature profile within the tube is uniform, then a relatively thick polycrystalline silicon deposit occurs on the wafers near the inlet portion and a relatively thin deposit occurs on wafers near the outlet portion. In order to remove this thickness variation, the furnace is typically profiled to provide a linear temperature increase from the inlet portion to the outlet portion. Since the rate of silicon deposition increases with deposition temperature, such a temperature profile results in a substantially uniform wafer-to-wafer deposition thickness.
SIPOS is typically deposited from a reactant gas mixture comprising SiH.sub.4 and N.sub.2 O. The volume ratio of N.sub.2 O to SiH.sub.4, known as .gamma., determines the oxygen content, and hence the resistivity of the deposited film. When forming SIPOS on a plurality of wafers, it is naturally desirable to achieve a similar film, from the standpoint of thickness and resistivity, on each wafer. However, the use of the constant temperature gradient (i.e., a linear temperature profile) which effects a substantially uniformly thick silicon deposit from a silane reactant gas, is not effective as a technique for forming a SIPOS film which is uniformly thick and uniformly resistive wafer-to-wafer. The extent of the thickness and resistivity non-uniformity is further accentuated in a high throughput, production-line environment. In such an environment, the number and size of the wafers within the deposition tube provide a deposition area which is relatively large, causing certain components of the reactant gas mixture to be significantly depleted before reaching the outlet portion of the tube.